1. Field of the Invention
The present invention relates to processing of semiconductor materials, and, more particularly, to laser assisted photochemical etching of semiconductors.
2. Description of the Related Art
Alloys of mercury telluride and cadmium telluride, generically denoted Hg.sub.1-x Cd.sub.x Te, are extensively employed as photosensitive semiconductors for infrared radiation detection. Indeed, Hg.sub.0.8 Cd.sub.0.2 Te has a bandgap of about 0.1 eV which corresponds to a photon wavelength of 12 .mu.m and Hg.sub.0.73 Cd.sub.0.27 Te a bandgap of about 0.24 eV corresponding to a photon wavelength of 5 .mu.m; and these two wavelengths are in the two atmospheric windows of greatest interest for infrared detectors. Extrinsic p-type Hg.sub.1-x Cd.sub.x Te has potential application in infrared focal plane arrays operating in the 10-12 .mu.m wavelength window.
Processing of Hg.sub.1-x Cd.sub.x Te to fabricate infrared detector devices requires etching patterns in Hg.sub.1-x Cd.sub.x Te substrates, and the current standard method of etching is ion milling which produces gross damage to the Hg.sub.1-x Cd.sub.x Te. A solution of 0.125% bromine in methanol is a standard etchant for lapping and polishing Hg.sub.1-x Cd.sub.x Te, but it is difficult to control and has not been used to etch patterns. A combination of ion milling followed by spray etching with 0.125% bromine in methanol to form large tapered vias in Hg.sub.1-x Cd.sub.x Te appears in U.S. Pat. No. 4,447,291. However, the known methods do not provide a damage-free etch that can be used to etch small features with vertical sidewalls.
CdTe and Hg.sub.1-x Cd.sub.x Te have been experimentally etched with lasers by photosublimation; see M. Rothschild et al, Laser Photosublimation of Compound Semiconductors 2 J. Mat. Res. 244 (1987) and C. Arnone et al, Laser Etching of 0.4 .mu.m Structures in CdTe by Dynamic Light Guiding. This approach focusses a laser beam onto a spot to locally vaporize the Hg.sub.1-x Cd.sub.x Te, and the pit or groove being etched forms a dynamic wave guide and keeps the pit or groove narrow. The sublimation leaves a tellurium residue lining the etched pit or groove and disrupts stoichiometry.
Thus the known methods do not selectively etch Hg.sub.1-x Cd.sub.x Te without stoichiometry disruption, substrate damage or photoresist use.
Other semiconductors such as silicon and GaAs are generally more robust than Hg.sub.1-x Cd.sub.x Te and have been etched successfully by various methods. For example, T. Donohue, Etching of Metal and Semiconductor Surfaces by Liquid-Phase Laser Photolysis in Beam Induced Chemical Processes (R. Gutfield et al eds, 1985 Mat. Res. Soc., Pittsburgh) reports etching of GaAs and copper by laser assisted wet etching in bromine and cerium chloride solutions, although the etching of the GaAs was at a very low rate.
Laser assisted etching of GaAs (or Si and InP) is reported in D. Podlesnik et al, Waveguiding Effects in Laser-Induced Aqueous Etching of Semiconductors, 48 Appl. Phys. Lett. 496 (1986) wherein a UV laser beam was focussed to a spot or patterned on a GaAs substrate immersed in a dilute nitric acid solution. The sidewalls of the grooves being etched provide a waveguide effect for the laser beam and deep narrow grooves were etched.
Laser projection dry etching of GaAs is reported in P. Brewer et al, Excimer Laser Projection Etching of GaAs, 49 Appl. Phys. Lett. 803 (1986). A laser beam was projected onto a GaAs substrate in a low pressure HBr atmosphere; the laser presumably dissociated the HBr into Br atoms which combined with the Ga and As to etch the substrate, producing volatile GaBr.sub.3 and AsBr.sub.3.
While various laser-assisted photochemical etches have been found for silicon and GaAs (and other III-V compounds), it is a problem to find etches applicable to Hg.sub.1-x Cd.sub.x Te or other II-VI compounds that avoid stoichiometry disruption, substrate damage, and use of photoresist.